At present, there are two basic fabrication routes for the production of Nb3Sn superconducting wire. The most common is the “bronze route” that features Nb filaments processed in a bronze (copper-tin) matrix. Wire produced by the bronze route is responsible for the majority of Nb3Sn wire production in the world. It is popular because, despite the need for intermediate anneals, the production process is rather straightforward and amenable to large lot sizes. For uses requiring higher superconducting critical current levels, the “internal tin” process is used to manufacture wire. In this process, the tin present is separate from copper present until the final heat treatment step. This process is used because the wire so made can deliver several times the supercurrent at high magnetic fields compared to wires made by the bronze process. This is because the internal tin process allows the creation of wire having more tin, and thus the capability to provide more Nb3Sn in the final wires' cross section.
An important performance measure for superconducting wire is the critical current density, Jc. Critical current density is defined as the maximum electric current a wire can carry divided by the cross sectional area (or some defined fraction of the cross sectional area) of the wire. A common form for expressing the critical current density is the non-copper critical current density, where the dividing area is all but the stabilizing copper. The Jc of a Nb3Sn superconducting strand made by the “internal tin” process (primarily a composite made of Cu, Nb, and Sn and/or their alloys) largely depends upon the fraction of Nb and Sn available in the wire cross section. Generally, the higher the fraction of Nb and Sn within the wire, the higher the fraction of the wire that can be converted to the Nb3Sn superconducting phase by strand heat treatment. As a result, modern designs for high Jc Nb3Sn strand made by the “internal tin” process consist of high Nb and Sn fractions, and a low amount of Cu.
Although a wire with the highest theoretical Jc would therefore be made of only Nb and Sn in a stoichiometric 3:1 atomic ratio (since this would maximize the amount of Nb3Sn in the cross section and minimize the fraction of non-superconducting Cu), in practice a certain amount of Cu is required in the cross section. The copper within the superconducting package or “subelement” serves several purposes, including the following:                1) Cu makes the wire easier to process because it has a hardness level between that of harder Nb and softer Sn. Cu is thus placed amongst the filaments, between the Sn core and Nb filaments, and between the subelements, to aid in the drawing process.        2) A small amount of Cu is needed to reduce the reaction temperature required for converting the Nb and Sn to Nb3Sn. This is desirable for obtaining Nb3Sn microstructures that result in a high Jc, and it is also desirable from a device manufacturing point of view.        3) The Cu also has an additional function. Cu between the Nb filaments serves as a path for diffusion of Sn, to allow the Sn source to be dispersed throughout the subelements and to all of the Nb filaments. Having adequate Sn locally available to all Nb filaments in a wire during heat treatment is important for reacting the Nb to Nb3Sn and providing a Nb3Sn microstructure that results in high Jc.        
Thus the problem of designing high current density Nb3Sn wires may be reduced to incorporating the optimum ratio of Nb, Sn, and Cu components in a package that can be fabricated and heat treated to produce a practically useable strand that is electrically stable as a supercurrent approaches its critical value (i.e., so that small non-homogeneities will not cascade the loss of supercurrent appreciable short of its upper bound value, known as a “quench”). It is desirable to design such a wire and provide a method for producing the same. More specifically, it is desirable to provide a unique summation and synergistic integration of all the concepts that produce the high critical current density.
Some past designs such as the “tube process” taught by Murase, U.S. Pat. No. 4,776,899, the disclosure of which is herein incorporated by reference in its entirety, have very high values of Sn wt. %/(Sn wt %+Cu wt %) within the diffusion barrier, and other designs have fine filaments with a low LAR as described infra. Still other designs have distributed diffusion barriers (diffusion barriers around each individual subelement separated by copper instead of a single diffusion barrier encasing all subelements). However, no previous designs have addressed all the issues that are critical for effectiveness and provided a solution to such issues. Field et al., U.S. Pat. No. 7,368,021, the disclosure of which is herein incorporated by reference in its entirety, provides a non-copper critical current density of about 3000 A/mm2 at 4.2K, 12 Tesla and about 1700 A/mm2 at 4.2K, 15 Tesla and provides an improvement of about tenfold over the initial internal tin superconductor wires and an approximately 50% increase over the prior art values of the late 1990's. The high performance of this conductor is achieved only when the subelement is about 60-180 microns at a typical final wire diameter of 0.5-2.0 mm. It would be desirable to provide similar performance for applications (e.g. particle accelerators) where magnetization and ac losses are beneficially kept to a minimum, and the subelement size is reduced to about 20-60 microns. Field et al., U.S. Pat. No. 7,368,021 provides superconducting wires having reduced performance of, for example, 2600 A/mm2 at 4.2K, 12 Tesla and 1300 A/mm2 at 4.2K, 15 Tesla at a subelement size of 45 microns. It is desirable to approach the 3000 A/mm2 at 4.2K, 12 Tesla value and 1700 A/mm2 at 4.2K, 15 Tesla value for a superconducting wire having a subelement size of about 20-60 microns.